For the best quality of reception of TV signals it is necessary to align the antenna with the TV transmitting station, and it is the usual rule in the design and construction of the masts upon which the antenna is to be mounted that some provision be made for the antenna to be rotated into alignment with the transmitting station.
Generally, it is required that the antenna should be set, in the rotational sense, individually for each transmitting station or source. Thus, it may be necessary, depending on local conditions, to reset the antenna every time a channel is changed. It is conventional for the TV viewer therefore to provide himself with a means for controlling the rotational setting of the antenna. The means may include an armchair control for an electric motor housed upon the mast, which, when energised, rotates the antenna.
In the conventional apparatus for rotating the antenna, a housing is clamped to the exposed top end of the fixed mast. The housing contains bearings for guiding and supporting a separate shaft upon which the antenna is affixed with U-bolts. The electric motor is contained within the housing, and acts to rotate the separate shaft when energised.
One of the main limitations with this conventional type of apparatus is that it is inconvenient to include independently rotatable antennas on the same mast. The problem is that any and every antenna attached to the separate shaft will rotate in unison when the shaft rotates. If, therefore, a household possesses two or three TV sets, and if each TV set were to have its own respective antenna attached to the said separate shaft, then, when the individual occupants are watching different channels (from different transmitters) only one of those two or three antennas would be aligned correctly with its transmitter at any one time.
Previously, the conventional way around this problem has been to provide two or three separate masts, which is very expensive. If the household possesses two TV sets, each with its own antenna, the problem will inevitably arise: because the reason for having two sets almost always is that different members of the household may thereby watch different channels. If one of the antennas on the mast is a TV antenna and the other is an FM radio antenna mounted on the same mast, even then the problem of different members of the household wanting to watch or listen to different programs will arise.